The rapid advances made in epitaxial growth on GaAs-type substrates has led to the development of a new class of electromagnetic wave detectors using the absorption of radiation around a wavelength λ corresponding to the electron transition between various energy levels within one and the same band or between the valence band and the conduction band. The diagram shown in FIG. 1 illustrates this type of transition.
The recent evolution in performance of this type of component is due in particular to the relatively easy production of semiconductor heterojunction multilayers in the standard system by MBE (molecular beam epitaxy), that is to say the GaAs/Ga(1-x)AlxAs. By adjusting the growth parameters, the thickness of the quantum wells and the percentage x of aluminum in the barriers imposing the confinement potential, it is possible to choose a narrow detection band (about 1 micron) centered on a given wavelength.
This type of structure has the advantage of providing very good sensitivity because of the discretization of the energy levels within the conduction bands of the photoconductor materials used.
Within the context of intersubband transitions, so that this type of transition is possible, it is necessary for the electric field of the incident electromagnetic wave to have a component along the direction of growth of the layers, i.e. along the direction D indicated in FIG. 1, this direction being perpendicular to the plane of the layers.
It has already been proposed to use coupling means of the diffraction grating type (cf. Goossen and Lyon, APL (1985), pp 1257-1259) for generating said perpendicular component, creating diffracted rays, especially lamellar (1D) gratings or steps for coupling only a single polarization of the light. However, crossed diffraction gratings are also known for coupling the various electric field components of an incident ray, such as the laser source LS as illustrated in FIG. 2. The matrix grating Rij diffracts the incident ray along both the direction Dx and the direction Dy.
The major drawback of the use of gratings is the wavelength and angular resonance associated with the increase in absorption, thereby limiting the use of these devices to a very narrow absorption window. These resonances are directly related to the periodic nature of the gratings. Thus, if it is desired to have a detector capable of detecting a range of wavelengths having a broader spectral band, solutions other than grating structures have to be sought.